This invention relates generally to an apparatus for detecting current and more specifically to an apparatus for detecting subsynchronous current in a high voltage transmission line.
At least some known high power transmission systems transmit a subsynchronous current along a transmission line. The subsynchronous current is a current which has a lower frequency than a normal transmission line frequency. The subsynchronous current is undesirable and when supplied to a power system generator may induce harmonic oscillations in a generator shaft that couples a prime mover to the generator. Harmonic oscillations are evident when series capacitor banks are utilized on a high voltage transmission line to increase a power flow capability. Electrical resonant frequencies of a series capacitor/transmission line system coupled a generator/shaft system may cause the harmonic oscillations to increase in intensity or magnitude. Continued operation with such oscillations and the associated shaft flexing may limit a useful life of the shaft.
To facilitate preventing subsynchronous current development, at least some known power transmission systems undergo extensive and expensive design reviews and testing prior to implementation. However, when a plurality of capacitor banks are coupled in series with the transmission lines, as is common in the proximity of generators, subsynchronous currents may develop when the capacitors are in service. Detecting appropriate levels of subsynchronous currents is difficult because the magnitude of troublesome subsynchronous current is very low compared to a nominal level of normal line frequency.
In one aspect, an apparatus for detecting currents in a three-phase power transmission system is provided. The apparatus includes a first detection circuit electrically coupled to a first phase of the three-phase transmission system, a second detection circuit electrically coupled to a second phase of the three-phase transmission system different than the first phase, and an event output switch electrically coupled to the first detection circuit and the second detection circuit and configured to actuate when a subsynchronous current on at least one of the first phase and the second phase exceeds a pre-selected subsynchronous current setpoint.
In another aspect, a series capacitor bank for a three-phase power transmission system is provided. The capacitor bank includes a first detection circuit electrically coupled to a first phase of the three-phase transmission system, a second detection circuit electrically coupled to a second phase of the three-phase transmission system different than the first phase, and an event output switch electrically coupled to the first detection circuit and the second detection circuit and configured to actuate when a subsynchronous current on at least one of the first phase and the second phase exceeds a pre-selected subsynchronous current setpoint.
In a further aspect, a three-phase power transmission system is provided. The transmission system includes a series capacitor bank, a first detection circuit electrically coupled to a first phase of the three-phase transmission system, a second detection circuit electrically coupled to a second phase of the three-phase transmission system different than the first phase, and an event output switch electrically coupled to the first detection circuit and the second detection circuit and configured to actuate when a subsynchronous current on at least one of the first phase and the second phase exceeds a pre-selected subsynchronous current setpoint.
In a still further aspect, a method for detecting subsynchronous currents in a power transmission system is provide. The method includes installing at least one detection circuit proximate at least one generator, coupling the detection circuit to a phase of a three-phase transmission system, monitoring the phase for a subsynchronous current, and activating an event output switch when the subsynchronous current exceeds a pre-selected subsynchronous current setpoint.